The present invention pertains to an apparatus for attaching a key member to a nuclear fuel assembly grid and detaching the same therefrom.
FIG. 4 depicts a known nuclear fuel assembly which includes a pair of top and bottom nozzles 1 and 2 arranged in vertically spaced relation to each other. A plurality of control-rod guide thimbles 3 are arranged so as to extend between the top and bottom nozzles 1 and 2, and fixedly secured thereto. A plurality of grids 4 are arranged on the vertically intermediate portions of the control-rod guide thimbles 3 so as to be vertically spaced from one another.
Each grid 4 is formed as follows. A plurality of straps 7, each of which is in the form of a thin strip sheet and has a plurality of slits 8 formed therein in longitudinally equally spaced relation from one another, are assembled perpendicularly to each other into a grid by mutually fitting the slits 8 thereof to define a number of grid cells 5. Dimples 9 and springs 10 for supporting a fuel rod 6 are formed on the walls defining each grid cell 5, and the fuel rod 6 inserted in the grid cell 5 is supported by being urged to the dimples 9 by the springs 10.
An assembly method of the nuclear fuel assembly thus constructed will next be described.
First, the grids 4 are arranged in equally spaced relation to each other so as to define a prescribed spacing therebetween. The control-rod guide thimbles 3 are then inserted into and fixed to predetermined grid cells 5 of the grids 4, which correspond to each other. The grids 4 are thus supported by the control rod guide thimbles 3. Thereafter, the fuel rods 6 are inserted into the other corresponding grid cells 5 of the grids 4 while bringing the fuel rods 6 into sliding contact with the springs 10 and the dimples 9. In this manner, the fuel rods 6 are fixedly arranged in corresponding grid cells 5 through the springs 10 and the dimples 9. After the completion of insertion of all the fuel rods 6, the top nozzle 1 and the bottom nozzle 2 are securely fixed to the opposite ends of the control-rod guide thimbles 3.
In the conventional assembly method as described above, inasmuch as the fuel rods 6 are inserted into the grid cells 5 of the grids 4 while bringing the fuel rods 6 into sliding contact with the springs 10 and the dimples 9, the outer peripheral surface of each fuel rod 6 may be scratched. Therefore, a method of inserting key members to deflect the springs 10 and subsequently inserting fuel rods while keeping the springs deflected is proposed in order to prevent the scratching. However, since the insertion of the key members is carried out by hands, the work has been labor-intensive, resulting in low efficiency.